Not sure though if .png should be the input format; most tools do offer a more "native" export format that could be used
x64sc -default -console -warp -limitcycles 100000 -exitscreenshot foo.png -autostartprgmode 1 foo.prg
Are you saying that everything is allright and nothing should be done?
Further, coding a "PETSCII displayer [...] because it’s embarassingly trivial" is not that trivial if you have no connection to coding at all (and not everyone's goal either; i don't see me (yet) coding a hires viewer) but again, the "Check your PETSCII submission" tool could solve that problem too. Output could be a high quality .prg file which again would make everyone happy.
However, i think a toolchain that is suitable for quick checking should involve VICE for running the provided prg and making a screenshot (this can be automated fully) and then some tool can be used to check the result for valid petscii.